1. Field of the Invention:
This invention relates to a system wherein an organic solvent containing extracted metal ions is mixed and contacted with a fluoride series stripping solution to deposit a fluorinated metal complex crystal and then the organic solvent, stripping solution and deposited crystal are individually recovered, and a process for operating such a system.
2. Description of the Prior Art:
Lately, solvent extraction is watched as a method for obtaining a high-purity metal or metal oxide, which is evaluated to be simple in the purification step and small in the energy consumption. And also, metals to be extracted include Mg, Ti, Cr, Mn, Fe, Co, Ni, Cu, Zn, Cd, Nb, Zr, Ta, Mo, W, In and the like.
In the solvent extraction method, the stripping (back-extraction) of the extracted metal ion has hitherto come into problem, which was solved by the use of a fluoride series stripping solution (an aqueous solution containing one or more of HF, NH.sub.4 HF.sub.2, NH.sub.4 F; see Japanese Patent laid open No. 57-42,545, No. 57-73,138, No. 57-73,141 and No. 57-85,943). Further, there is proposed a crystallization apparatus of an inverted frustocone structure for use in the solvent extraction in Japanese Patent laid open No. 58-81,402. However, such a crystallization apparatus has been used only in a laboratory scale and has not yet been put to practical use in an industrial scale.
The inventors have made various studies in order to industrialize the crystallization apparatus disclosed in Japanese Patent laid open No. 58-81,402 and first developed an industrial scale apparatus having a crystal production capability of 45 tons/month and being able to conduct the continuous service for a long time or period, which was filed under the title of "Crystallization Apparatus for Metal Stripping" at the same time, or on the same date of the filing of this application.
In the operation of such an industrial scale apparatus, however, it is not clearly described to maintain the concentration and composition of the stripping solution circularly used at predetermined control ranges. And also, such an apparatus is not necessarily sufficient to perform the adjustment of the above concentration and composition. For instance, when the kind and position of chemicals fed to the circulating flow of the fluoride series stripping solution are inadequate, there are unfavorably caused troubles such as deposition of crystal inside the pipe or pump, clogging of the pipe or pump and the like. In the conventional metal separation system shown in FIG. 7, the above troubles are often produced during the operation as mentioned in the following comparative example, which is obliged to stop the operation of the system.
Further, the step of circulating the organic solvent in the solvent extraction method using the above apparatus consists mainly of extraction and stripping (back-extraction) stages. At these two stages, the working temperature is usually adjusted to be made relatively high or not to be excessively lowered in order to enhance the extraction and back-extraction efficiencies.
If the working temperature is too low, the viscosity of the organic solvent increases and the handling thereof is somewhat difficult, and also the extraction or back-extraction efficiency and rate lowers. Particularly, as the stripping temperature in the metal stripping step becomes higher, the back-extraction rate of metal ion is high and also the crystallization conditions for the resulting fluoride compound become advantageous.
On the other hand, if the working temperature for the organic solvent is too high, the deterioration of the organic solvent comes into problem. That is, the deterioration of the organic solvent results from a certain chemical change or the like and appears as a reduction of metal extraction rate or the like. Especially, when the organic solvent is continuously contacted with a strong acid or stripping solution, the deterioration is caused by hydrolysis or the like even at room temperature. And also, the degree of deterioration becomes conspicuous as the contacting temperature rises. In this connection, Japanese Patent laid open No. 55-18,512 merely discloses that the deterioration of organic solvent for the extraction of mineral acid (consisting of a neutral phosphate ester extractant and an aromatic hydrocarbon diluent) is suppressed by restricting the working temperature to not more than 30.degree. C.